1. Field of the Invention
This invention belongs to the field of processes for producing polyethylene film products and, in particular, to the processes for producing improved shrink wrap from polyethylene homopolymers and copolymers.
2. Description of the Prior Art
Low density polymers, i.e., generally considered as those polymers having a density of less than about 0.94 gm/cm.sup.3, have been modified by many different post finishing methods in order to achieve certain physical properties which are desirable for some types of products. Low density polymers such as polyethylene homopolymers and copolymers have, for example, been oriented unaxially and/or biaxially in order to gain higher tensile strength and to gain a greater degree of clarity and transparency. Likewise, it is known that irradiation of such polymers causes crosslinking of polymer chains thereby making the resins more "form-stable" and capable of retaining a greater degree of transparency.
Post finishing operations of orienting and irradiating have also been used to produce a polyethylene polymer which has "shrink" energy and is intended for use in the fabrication of containers, closures, squeeze bottles, toys, film packaging materials, flexible bags, etc. For example, U.S. Pat. Nos. 3,144,398, and 3,144,399 disclose, respectively, a process for producing and a shrinkable polyethylene product prepared by cold stretching polyethylene and then irradiating the stretched polyethylene with a dosage of from about 20.times.10.sup.6 to about 200.times.10.sup.6 rep. (one rep. being defined as that amount of nuclear radiation which dissipates 93 ergs of energy per gram of tissue, i.e., approximately equal to the amount of energy that would be dissipated by a one roentgen X-ray beam in a gram of tissue). While these disclosures are addressed primarily to processes for preparing low density polymer sheet used for forming shaped articles, similar techniques have been considered for use in the preparation of polymer films.
U.S. Pat. No. 3,022,543 describes a method for preparing polyethylene film having, inter alia, a strong biaxial shrinking force and greatly increased tensile strength. The film is prepared by stretching a tube of extruded film in at least one direction while the film is a little above room temperature cooling the "tube" of film, irradiating the film with a dose of at least 2.times.10.sup.6 rep. reheating the irradiated tube while in a bath of inert liquid to a temperature of at least 65.degree. C., laterally stretching the tube by the "bubble" method, and stretching the tube, again, longitudinally, at least 100% while in the reheated state followed by cooling the film prior to release of the stretching.
In U.S. Pat. No. 3,663,662 a cross-linked, oriented film of a blend of polymers is described which is both heat-shrinkable and heat-sealable so as to be usable on automated packing equipment. The blend includes about 70% to 85% by weight of a low density polyethylene homopolymer or copolymer having a density of from 0.91 to 0.93 gram/cc at 25.degree. C., the balance of the blend being a high density polyethylene homopolymer or copolymer having a density of 0.94 to 0.98 gram/cc at 25.degree. C. The film formed therefrom is capable of shrinking at least 15% in each direction in the plane of the film at a temperature of 100.degree. C., a shrink tension of at least 200 psi at 100.degree. C. and a zero strength temperature of at least 175.degree. C. in each direction. This film is obtained by forming a self-supporting film of the polymer blend indicated above, heating the film to a temperature of 90.degree. C. to 115.degree. C., stretching the film by a factor of at least five in each of two mutually perpendicular directions in the plane of the sheet, cooling the film under tension, and irradiating the film for a period sufficient of raise the zero strength temperature to 175.degree. C.
The object of each of the foregoing processes can generally be considered as the production of a film having increased overall shrink energy. Higher overall shrink energy, however, may be achieved only at the expense of other desirable goals and properties, e.g., by introducing additional complexity to the process and/or reducing film strength.